Electrically operated locks are well known in the art. For example, U.S. Pat. No. 848,256 issued Mar. 26, 1907 discloses such an electrical lock for a railway truck while U.S. Pat. No. 1,283,845 issued Nov. 5, 1918 shows a similar lock employing push-buttons for a safe.
Considerably more recently, U.S. Pat. No. 3,691,396 issued Sept. 12, 1972; U.S. Pat. No. 3,751,718 issued Aug. 7, 1973 and U.S. Pat. No. 3,831,065 issued Aug. 20, 1974 relate to electronic push-button combination locks employing solid state circuitry.
All of these locks operate essentially on the principle of actuating selected ones of keys on a keyboard in a given sequence to generate an electrical signal which in turn will throw a solenoid actuated bolt to unlock a door. Many of these known circuits further include means for preventing generation of the unlocking signal in the event a wrong key is punched as well as appropriate resetting means for locking the door.
In the specific case of trucks hauling trailers or of the van type in which access to cargo space in the truck is by way of a rear upwardly sliding door or equivalent access door, such combination push-button type electric locks are very useful. Thus, the operating keyboard can be located on the dashboard of the truck and the driver when he wishes to gain access to the rear need only punch out the appropriate combination and then go to the rear of the truck and open the door. On the other hand, certain problems can arise with such an arrangement when utilized on vehicles. For example, a driver may punch out the correct combination while in the cab and then for some reason or other be delayed in walking around to the rear to open the door. In the event such delay is for an unusually long period of time, as might occur should the truck driver have a heart attack, the door lock would be opened and unauthorized personnel could then gain access.
In other instances, it is conceivable that an unauthorized person could simply by trial and error attempt to open the rear door by arbitrarily pushing certain keys on the keyboard. Finally, where the solenoid operated bolt locking mechanism itself is located adjacent the door at the rear of the vehicle, while the keyboard itself is located on the dashboard, wire connections from the keyboard to the lock are necessary and it is possible that an unauthorized person may attempt to tap into the wires leading to the locking mechanism and generate a signal for operating the same.